1. Field of the Invention
The present invention relates to a fluorescent layer and its preparation method and uses. In particular, the present invention relates to a fluorescent layer useful in light emitting diodes and its preparation method.
2. Descriptions of the Related Art
With the awareness of saving energy and protecting the environment, white light emitting diodes (LEDs) have become the most anticipated emerging product and have gradually replaced traditional lighting equipments because of advantages including a smaller size (in response to the trend of miniaturization), low power consumption (one eighth to one tenth of conventional light bulbs, and half of fluorescent lamps), a long lifespan (more than 100,000 hours), low heat generation (low heat radiation), and excellent response time (capable of high frequency operation). White LEDs can solve many problems that incandescent bulbs could not solve, and thus, have become a new hope of lighting in the 21st century. In addition, the white LED is a “green lighting source” because it is both power-saving and eco-friendly.
White LEDs that were developed in the past consisted of a plurality of LEDs with various light emitting wavelengths. However, such a device is limited in application to light emitting devices requiring high luminance due to a large volume, poor luminous efficiency and uneven color mixing. In principle, current white LEDs consist of a single wavelength light source (LED chip) and at least one fluorescent material excitable by the light source. The fluorescence emitted from the excited fluorescent material is mixed with the light emitted from the light source (which is not absorbed by the fluorescent material) to form white light. For the structure of current white LEDs, the fluorescent material is generally mixed with a packaging material such as an epoxy resin to form a package, and then, a light source is covered with the package, i.e., forming a fluorescent layer on the light source to provide the white LED.
Nonetheless, the above light emitting device, when used for a time period, often has the problem of aging (etiolation) because the epoxy resin therein is over-crosslinked due to the absorption of ultraviolet light or heat generated by the diode. This aging problem lowers the luminous efficiency of the light emitting devices significantly. In addition, the fluorescent material in the light emitting layer also has effects such as heat exhaustion or thermal quenching as the temperature increases.
The industry has improved the heat dissipation performance of the light emitting device to lower the extent of heat exhaustion of the fluorescent material and to slow down the aging (etiolation) of the epoxy resin. U.S. Pat. No. 7,361,938 discloses a fluorescent plate member to improve the problem of heat dissipation, wherein the fluorescent plate is prepared directly by thermally pressing YAG fluorescent powders at about 1700° C. The fluorescent plate provided by the foregoing means may prevent the fluorescent resin layer from aging and lower the light scattering, but it requires a large amount of fluorescent material and repeated high temperature thermal treatments, thus, increasing the cost.
Based on the needs mentioned above, the present invention provides another fluorescent layer and the preparation method thereof, which can improve the heat dissipation of LED, and can rid of the aging (etiolation) problem because the fluorescent layer of the present invention is free of an organic resin. The final product exhibits characteristics of stable light emission, a long lifespan and high durability. Moreover, the present invention uses a cheaper calcining powder, which can be sintered at a lower temperature, to reduce the manufacturing cost and lower the process difficulty, thus, overcoming many problems encountered in the prior art.